The study of the developmental regulation of tooth number and shape is important for understanding human disease. Mammalian tooth development is controlled by signaling between the oral epithelium and the neural crest-derived ectomesenchyme. These interactions are mediated by several signaling pathways, including those activated by Fibroblast Growth Factors (FGFs) via binding to FGF receptor- tyrosine kinases. Members of the Sprouty family modulate FGF signaling by antagonizing it downstream of the receptor. In my previous work, I have shown that inactivation of Spry2 or Spry4 results in the formation of supernumerary teeth in the diastema, a normally toothless region between the incisors and molars of mice. These supernumerary teeth result from hypersensitivity of the odontogenic epithelium or mesenchyme to FGF signaling. In this application, I will pursue the observation that inactivation of multiple Sprouty alleles has profound effects on incisor development. Embryos that are null for Spry4 and heterozygous for Spry2 have supernumerary incisors in the maxilla and remarkable tusk-like incisors in the mandible. Embryos that are null for two Sprouty genes also develop supernumerary incisors, although the morphology of these incisors varies depending on the genotype. The first group of experiments will test the hypothesis that the tusk-like incisor phenotype is caused by effects on stem cells in the adult incisor. I will analyze the role of FGFs and other signaling molecules in regulation of incisor stem cells. The second group of experiments in this proposal is aimed at elucidating the normal mechanisms that control the number and morphology of incisors, as well as the role of Sprouty genes in these processes. Results from these studies will enhance our understanding of the molecular pathways that control epithelial-mesenchymal interactions during tooth organogenesis and may help lead to innovative treatments for patients with dental defects, including even the possibility of bioengineering new teeth. The studies proposed here employ a genetic approach to determine the functions of the Sprouty (Spry) genes in mouse tooth development. Public health implications: By studying the functions of the Sprouty gene family, we will learn more about how teeth normally develop and will understand more about the basis for dental abnormalities. We will also study the role of adult stem cells in teeth, which may help to lay the groundwork for efforts to build new teeth.